The present invention relates to transmission systems employing optical fibres.
Presently known systems are of two types: real time systems, in which either the available pass band is divided into channels of sufficient width to allow the transmission of the useful band, or lights of different colours are used at emission and filters at reception; shared time systems, in which each channel is coded then transmitted sequentially on the line, the transmission cadence being much higher than the highest frequency of the useful band to be transmitted. Combinations of the two types are possible.
Real time systems are affected by the non-linearities, at all levels, of the transmission channel, which provoke parasitic beats. Shared time systems raise serious problems of synchronisation and decommutation.
The present invention relates to a system in which the spectrum of the information to be transmitted is spread out over the whole of the available pass band at emission. To this end, a random signal is modulated by the information to be transmitted.
At reception, the signal received is intercorrelated with a copy of the random signal used at emission, recreated locally. In fact, "pseudo-random" codes are used. This system is known as "spectrum spread" system.
Advantages thereof are numerous:
Emission and reception may be simultaneous on the same transmission channel;
The non-linearities do not affect the results a great deal;
The problems of format synchronisation do not exist.
Furthermore, in the transmission of light pulses by optical fibres, the information to be transmitted modulates the light intensity of a source (electroluminescent diode, laser). The modulated light is injected into one or more fibres which guide it with a slight attenuation until it is used, when it is converted, again, into electrical energy by a detector (PIN photodiode or cascade photodiode).
The advantages are numerous and important: electrical insulation, no radio-frequency radiation, very little attenuation (presently 2 db/km), and very large pass band (presently 2 GHz).